[Source: NOAA Climate Prediction Center, http://www.cpc.ncep.noaa.gov/ ]
The daily North Atlantic Oscillation (NAO) index is constructed by properly projecting the daily (00Z) 500mb height anomalies
over the Northern Hemsiphere onto the loading pattern of the NAO.
The daily NAO index and its forecasts using MRF and Ensemble mean forecast data are available for the previous 120 days and
they are normalized by standard deviation of the monthly NAO index from 1950 to 2000. A 3-day running mean is applied for
the forecast indices.
Monthly and seasonal (JFM) mean NAO index data from 1950 are also available. See: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao_index.html